This invention relates to an array of thermocouples, hereafter referred to as "thermopile," that is used to produce electrical power by means of the principle commonly known as thermoelectricity, wherein a portion of heat traversing solid (or liquid) materials is converted to electricity and to the manufacture of such a thermopile. More particularly, the invention relates to a semiconductor thermopile and a method for its manufacture.
The production of electrical power from heat by means of thermoelectricity is a well known principle that is widely used in specialized applications. A device that produces electrical power by this means is known as a thermoelectric generator. A thermoelectric generator generally consists of a source of heat, either as an integral part or separate of the generator, a thermopile that is thermally coupled on one side to the heat source and on the other side to a heat sink, thermal insulation that minimizes extraneous heat losses and various structural members, including the outer case of the generator. A cold side heat exchanger that rejects waste heat is thermally connected to the outside of the thermoelectric generator and frequently is an integral part of the generator, especially when heat is rejected by radiation and/or by natural convection.
One prior art type of semiconductor thermopile consists of one or more electrically interconnected thermocouples, either in a series or in a parallel or in a series-parallel arrangement, wherein each thermocouple usually consists of two thermoelements, one possessing n-type and the other possessing p-type conductivity characteristics. N-type and p-type conductivity refer to common solid state physics terminology wherein the former conductivity refers to materials in which electrical conduction occurs primarily as a result of the movement of negative carriers, such as electrons, and wherein the latter conductivity refers to materials in which electrical conduction occurs primarily as a result of the movement of positive carriers, such as holes. The thermoelements of such themocouple are electrically interconnected at one side by means of an electrically conductive bridge or electrode. In the case of a single thermocouple, the electrical output of the thermocouple is obtained from approximate contacts placed at the opposite, non-interconnected ends of the thermocouple. The output contacts of the thermocouple are also electrically conductive and may be electrically interconnected to other thermocouples in the thermopile, if the thermopile consists of more than one thermocouple; in the case of a multi-thermocouple thermopile, the output terminals are connected to one ouput contact of each of at least two thermocouples. The thermopile may consist of a number of physically separated thermocouples that are electrically interconnected or it may consist of a monolithic matrix which contains more than one thermocouple and in which the thermocouples are mechanically or metallurgically attached to each other through an intermediate layer of electrical insulation and are electrically interconnected.
The dimensions and configuration of thermoelements in a thermocouple, and the number of thermocouples in a thermopile, are determined by detailed design considerations that include the electrical output power and voltage required of the thermopile, the amount of heat available and/or required by the thermopile, and the operating temperatures of the thermocouples in the thermopile. The efficiency of a thermoelectric generator is generally proportional to the temperature difference across the thermocouples of its thermopile and also to a quantity known as the "figure-of-merit" that depends on certain basic properties of the thermoelectric material used in the generator. For a given thermoelectric material and operating temperatures, the power produced by a thermoelectic generator is approximately proportional to the total cross-sectional area of the thermoelectric material contained by it; the output voltage of the generator is proportional to the number of series-connected thermocouples into which the thermoelectric material is subdivided. The required ratio of individual thermoelement length to cross-sectional area generally increases with decreasing values of power output because the desired voltage output values of a thermoelectric generator are relatively independent of the power output required of it.
Thus, for example, most thermoelectric generators that produce electrical power at power levels in the tens or even hundreds of watts require output voltages on the order of a few volts to a few tens of volts. Smaller thermoelectric generators, such as might be implanted in a human body to power a heart pace maker, for example, that produce electrical power in the microwatt or milliwatt power range generally are still required to produce voltages in the several-volt range. A reduction in the required power output by several orders of magnitude thus usually results in only a relatively modest reduction in the required voltage output. This of course means that although the total number of thermocouples in a generator that produces very small amounts of electrical power is nearly the same as in a generator that produces much more power, the total amount of thermoelectric material is significantly reduced. As a consequence, the ratio of the length to cross-sectional area of individual thermoelements in the former case is of course much more extreme than in the latter case. In fact, in the case of most commonly used thermoelectric materials, this ratio frequently becomes so extreme at power levels in the microwatt and low milliwatt range that it is not possible to manufacture optimum thermopiles that directly produce the voltages required for many applications.
The reason why it is not possible to manufacture such thermopiles with most commonly used thermoelectric materials is that these materials frequently do not have sufficient mechanical strength to allow their failure-free manufacture into thermoelements with the extreme ratio of length to cross-sectional area required of thermopiles that produce high voltages at low values of power output. As a result, it is usual to manufacture thermopiles that produce either much less than the required voltage at the desired power output or a much greater than required power output at the desired voltage. In both cases, it is the conversion efficiency of the thermoelectric generator that is penalized.
In the former case, it is necessary to use a voltage converter to convert the low direct output voltage to the value desired. In the conversion process, power is lost in the voltage converter and therefore the thermopile must be designed to produce more than the final required power in order that adequate power be available after voltage conversion. In the latter case, in which a much greater than required power output is produced, conversion efficiency is lost because a proportionately greater amount of heat is required to power the generator. In addition to a loss in conversion efficiency, in both cases the generator size must be increased to accommodate the greater fuel requirements; generator cost very likely also increases for the same reason.
Commonly used heat sources in thermoelectric energy conversion consist of burners that burn various fossil fuels, radioactive materials that produce heat upon their decay, either focused or unfocused solar heat, heat produced by nuclear reactors and the waste heat of a variety of engines. The thermopile used in conjunction with any of such conventional heat sources operates between the temperature of the heat source and the temperature of a heat exchanger that rejects waste heat into ambient environment of the thermoelectric generator. The cold side heat exchanger operates at a temperature lower than that of the heat source and higher than that of the ambient environment. The thermopile of most low power thermoelectric generators, those that produce electrical power in the microwatt or the milliwatt range, is generally a monolithic matrix wherein the individual thermoelements thermocouples are either mechanically or metallurgically attached to each other through intermediate layers of electrical insulation. The electrical interconnects between thermoelements and thermocouples are generally located in the two extremities of the thermopile. The electrical output leads of the thermopile emanate from the two end-most thermocouples in the thermopile. The reason for the inclusion of the thermoelements and thermocouples in a monolithic matrix is that because of the extreme ratio of length to cross-sectional area, individual thermoelements tend to be extremely weak mechanically. The cross-sectional area of individual thermoelements is generally extremely small because the thermopile size constraints limit the thermoelement height. Even with this type of thermopile construction, there is usually a limit as to how small thermoelement cross-sectional areas can be made with most commonly used thermoelectric materials. As already discussed, this limit restricts either the maximum obtainable voltage at a given level of power output or the minimum obtainable power output at a given voltage. A typical low power thermoelectric generator that uses radioisotopes as a heat source is illustrated in FIG. 1. It should be noted that radioisotopes are probably the most widely utilized heat source in totally self-contained, long-life miniature thermoelectric generators that produce power in the range of microwatts and milliwatts.
Basically, every material is a thermoelectric material in that it possesses the three properties commonly used to define a thermoelectic material: the Seebeck coefficient, electrical resistivity, and thermal conductivity. As already mentioned, the conversion efficiency obtainable from a thermoelectric material is proportional to a quantity known as the "Figure-of-Merit." The Figure-of-Merit of a material is defined as the quotient of the square of the Seebeck coefficient and the product of electrical resistivity and thermal conductivity. As a result of extensive research, it has been found that as a general class, the most efficient thermoelectric materials are extrinsic semiconductors. Although metals and insulators, as well as intrinsic semiconductors, can all be used to convert heat to electrical power, the conversion efficiencies obtainable with those materials are significantly lower than the conversion efficiencies obtainable with extrinsic semiconductors. Even in the broad class of extrinsic semiconductors, wide divergencies exist between the ability of different materials to convert heat to electricity, i.e. wide differences exist in the figures-of-merit, hence conversion efficiency obtainable with different extrinsic semiconductors.
The most commonly used extrinsic semiconductors in thermoelectric energy conversion are compounds and alloys that include Bismuth and Tellurium, Lead and Tellurium, and alloys of Silicon and Germanium. Most of the thermoelectric materials based on these material combinations are inherently intrinsic; extrinsic characteristics are obtained by these materials as a result of doping with appropriate impurities to yield n-type and p-type conductivity. Occasionally, although infrequently, use is made of metals in the form of very fine wires, to produce electrical power thermoelectrically. Although metals are relatively inefficient thermoelectric materials, they do afford the obtainment of a high ratio of thermoelement length to cross-sectional areas and thereby enable the obtainment of a high direct output voltage from thermoelectric generators designed to produce low values of power output. The conversion efficiency, however, is so poor compared to extrinsic semiconductors that a significant penalty is paid in terms of required fuel loading and/or device size. For this reason, there is an obvious advantage to the use of the just-mentioned commonly used compounds and/or alloys in the production of thermopiles designed to produce small amounts of electrical power, even though as explained such thermopiles usually are unable to produce high direct output voltages.
Each of the three general class of extrinsic thermoelectric materials commonly used in the production of electrical power from heat has traditionally occupied a definite place in the overall field. The figures-of-merit and maximum operating capabilities of each group of materials differ widely. Bismuth-Telluride and compounds and alloys that utilize it are basically low temperature materials that cannot reliably be operated at temperatures in excess of some 250.degree. to 300.degree. C. It does, however, possess the highest known figure-of-merit of any material at low temperatures, those in the neighborhood of room temperature. Lead-Telluride and compounds and alloys that utilize it are relatively higher temperature materials that can be operated up to about 500.degree. to 600.degree. C. The figure-of-merit of materials based on Lead-Telluride are somewhat lower than those materials based on Bismuth-Telluride. Silicon-Germanium alloys are capable of operation up to about 1000.degree. C. Their figures-of-merit are somewhat lower than those of materials based on Lead-Telluride. In general, because the conversion efficiency of a thermoelectric device is proportional to both the figure-of-merit and the temperature difference across which the thermoelectric material is operated, it is found that the three general groups of extrinsic semiconductors enable the manufacture of thermopiles and thermoelectric generators that exhibit relatively comparable conversion efficiencies.
When the conversion efficiencies of thermoelectric generators are optimized, it is usually found, relatively independent of the thermoelectric material used, that the hot side temperature of the thermopile at optimum generator efficiency is a function of generator power level such that the lower the power desired from the generator, the lower the hot side temperature at which generator efficiency optimizes. In general, the conversion efficiencies of thermoelectric generators designed for the low milliwatt power output range possess thermopile hot side temperatures of some 300.degree.to 400.degree. C. or less at optimum values of conversion efficiency. In the high microwatt power output range, thermopile hot side temperatures that correspond to optimum efficiency are generally less than 200.degree. C. Because alloys and compounds based on Bismuth-Telluride and those based on Lead-Telluride, but especially the former, possess the highest known figures-of-merit of all commonly used thermoelectric materials at such temperatures, it may seem obvious that it is these materials that should be used in low power thermoelectric generators. Based on considerations that only address themselves to the conversion efficiency of a thermoelectric generator, this conclusion is inescapable and, in fact, forms the basis of much of the state-of-the-art of low power thermoelectric generator technology. Although the conversion efficiency of thermoelectric generators that use alloys and compounds of Lead and Bismuth-Telluride are relatively high, at the very lowest power levels, in the low milliwatt and microwatt power range it is not generally possible to obtain sufficiently high direct output voltage values to satisfy the requirements of most applications of the generators. Accordingly, it is not uncommon to over-design these generators from the standpoint of electrical power produced with the view of using either a voltage converter to enchance the output voltage or to obtain the voltage directly at the higher power level.
As already explained above, the end result of such a procedure is a power conversion system that is not really optimized either for efficiency or size. If a voltage converter is used, the reliability of the overall power conversion system is also penalized because of the addition of an electronic component in the system. The reason that it is not possible to obtain an adequate voltage output from low power thermoelectric generators that use alloys and compounds of Lead and Bismuth-Telluride as their thermoelectric material, is that these materials are relatively brittle and weak and therefore do not lend themselves to manufacturing procedures that enable the production of thermoelements with sufficiently extreme ratios of length to cross-sectional area.